In milling certain types of materials it is often necessary or desirable to have a positive control of the atmosphere within the mill at all times. For example, readily oxidizable materials such as aluminum, titanium, magnesium, lithium and fine powders of many compositions are combustible or even explosive under certain conditions or they may be contaminated by the presence of air. In milling such materials the control of the atmosphere must extend to charging and discharging of the mill without opening the mill to air.
In designing apparatus to be used for metals and other powders special consideration must be given to valve design when the powders will come in contact with a valve because the powders will find their way into valve parts and render the valve inoperable.
The problems encountered in milling powders are particularly troublesome in the mechanical alloying of readily oxidizable metals such as aluminum, titanium, magnesium, lithium, and rare earths such as cerium. Mechanical alloying has been described in detail in the literature and in patents. U.S. Pat. Nos. 3,740,210, 3,816,080 and 3,837,930, for example, involve the mechanical alloying of aluminum alloys and other composite materials containing aluminum. In the practice of mechanical alloying the components of the product are charged in powder form into a high energy milling device such as a ball mill where, in an environment free of or reduced in amount of free or combined oxygen, the powders that are dry or substantially dry are ground down to a very fine size initially, prior to particle agglomeration in the latter stages of the process. This initial grinding increases the total surface area of the metallic powders significantly. Since any freshly exposed surface is not oxidized, it is very hungry for oxygen to the extent that the powders in this condition will burn and/or might explode spontaneously if exposed to air. Thus, any port in the mill, for example, for charge or discharge of powders, is a source of potential danger from the standpoint of the quality of the product produced and the possibility of a fire and/or an explosion.
In batch-type grining mills it has been conventional to provide charging devices which are normal to the mill. That is, when the mill shell is positioned for loading, the charging device is in a vertical position with the charge port at the top of the shell. In U.S. Pat. No. 4,679,736, loading means in an improved design is disclosed. In the improved design the loading means is in a vertical position when the mill shell is rotated so that the loading means is on the side of the mill with unencumbered, gravity-dependent entry for the feed material into the shell. Also disclosed is apparatus for utilizing the disclosed charging design in mills in which a protective atmosphere is required. The present invention is particularly effective with the positioning design of the loading means of the aforementioned disclosure.
The present invention involves a valve system for charging rotary mills in a manner which will protect the charge material and the environment in the mill during loading of the mill, while at the same time it is designed to minimize the problem of wear of the valve parts during operation of the mill.
The charge system of the present invention can be incorporated into existing batch-type rotary mills, permitting them to be charged and operated under protective conditions.